Diffusion of fluorescently labelled molecules (proteins, lipids, carbohydrates) on cell surfaces is measured by the technique of fluorescence photobleaching recovery (FPR). Generally fifty to ninety percent of the detectable membrane proteins (cell surface receptors for antigens, lectins and hormones) are free to move in the membrane with diffusion coefficients of about 10 sq. cm./sec. The remaining are immobile. Lipid probes usually have diffusion constants of about 10 to the minus 8th power sq. cm./sec, with essentially all the probe mobile in the membrane. The technique has recently been applied to the question of membrane asymmetry between apical and basolateral surfaces in epithelial cell cultures. One surface of the cell is labeled with a fluorescent probe and movement of that probe across the tight junction to the other surface is assessed. Other techniques to measure movement of molecules in membranes are fluorescence polarization, fluorescence energy transfer and fluorescence life time heterogeneity analysis. Those experiments lead to the notion that lipids in cell membranes are not homogeneously dispersed, but are organized in domains of different fluidity.